Scaling a glyph
This section describes how glyph outlines are scaled from the master size stored in the font file to the size requested by an application.
Device space
Whatever the resolution of the em square used to define a glyph outline, before that glyph can be displayed it must be scaled to reflect the size, transformation and the characteristics of the output device on which it is to be displayed. The scaled outline must describe the character outline in units that reflect an absolute rather than relative system of measurement. In this case the points that make up a glyph outline are described in terms of pixels.
Intuitively, pixels are the actual output bits that will appear on screen or printer. To allow for greater precision in managing outlines, TrueType describes pixel coordinates to the nearest sixty-fourth of a pixel.
Converting FUnits to pixels
Values in the em square are converted to values in the pixel coordinate system by multiplying them by a scale. This scale is:
where pointSize is the size at which the glyph is to be displayed, and resolution is the resolution of the output device. The 72 in the denominator reflects the number of points per inch.
For example, assume that a glyph feature is 550 FUnits in length on a 72 dpi screen at 18 point. There are 2048 units per em. The following calculation reveals that the feature is 4.83 pixels long.
Display device characteristics
The resolution of any particular display device is specified by the number of dots or pixels per inch (dpi) that are displayed. For example, a VGA under OS/2 and Windows is treated as a 96 dpi device, and most laser printers have a resolution of 300 dpi. Some devices, such as an EGA, have different resolution in the horizontal and vertical directions (i.e. non-square pixels); in the case of the EGA this resolution is 96 x 72. In such cases, horizontal dots per inch must be distinguished from vertical dots per inch.
The number of pixels per em is dependent on the resolution of the output device. An 18 point character will have 18 pixels per em on a 72 dpi device. Change the resolution to 300 dpi and it has 75 pixels per em, or change to 1200 dpi and it has 300 pixels per em.
Figure 1-8 18 point figure 8 at 72 dpi, 300 dpi and 1200 dpi
Displaying type on a particular device at a specific point size yields an effective resolution measured in pixels per em (ppem). The formula for calculating pixels per em is:
On a 300 dpi laser printer, a 12 point glyph would have 12*300/72 or 50 ppem. On a typesetter with 2400 dpi, it would have 12*2400/72 or 400 ppem. On a VGA, a 12 point glyph would have 12*96/72 or 16 ppem. Similarly, the ppem for a 12 point character on a 72 dpi device would be 12*72/72, or 12. This last calculation points to a useful rule of thumb: on any 72 dpi device, points and pixels per em are equal. Note, however, that in traditional typography an inch contains 72.2752 points (rather than 72); that is, one point equals .013836 inches.
If you know the ppem, the formula to convert between FUnits and pixel space coordinates is:
An em_coordinate position of (1024, 0) would yield a device_pixels coordinate of (6, 0), given 2048 units per em and 12 pixels per em.